Local oscillators are used in various electronic circuits to generate a signal to be combined with a communication signal. Such is the case, for example, in direct-conversion receivers and transmitters, in which the communication signal is converted in a single step between communications frequency and base-band frequency. In the case of a radio receiver, a radio signal received may be directly down converted to In-phase (I) and Quadrature-phase (Q) signals and, in the case of a radio transmitter, a radio signal to be transmitted may be directly up converted from In-phase (I) and Quadrature-phase (Q) signals. Such direct-conversion receivers and transmitters enable a high degree of integration of the circuits, for example by avoiding the need for band-pass filters, as are required in heterodyne receivers and transmitters.
It is important to minimise the effect of spurious signals also being combined with the communication signal, such spurious signals appearing as a consequence of ‘leakage’, the propagation of unwanted, parasitic signals generated by the local oscillator and other circuits in the communications apparatus.
A direct-conversion receiver is described in U.S. Pat. No. 5,530,929, including a local oscillator that is connected to a first processing unit that multiplies the output frequency of the local oscillator by a factor M. The first processing unit is operatively connected to a second processing unit in which the output signal of the first processing unit is divided by a factor N. M and N are both integer numbers and examples are given with M=3 and N=2 and with M=2 and N=3. The output signal from the second processing unit is supplied to I and Q mixers where it performs a homodyne conversion to base-band of the incoming RF communication signal, at least the second processing unit being integrated with the mixers to reduce propagation of spurious signals.